Step 8: The Hardware: an ultralight solar charge controller

The solar charger has four main subsystems. I'm going to spend a few minutes going over them. Feel free to skip this page if you care more about the "hot to make this" than the "why this works," as it runs a little long.

The solar cells. The cells we are using are flexible, thin-film plastic cells. They convert sunlight into electricity. In our case, our solar array puts out 3 volts and 150 milliamps in full sun, and less than that in cloudy, hazy conditions. The voltage and current provided by the cells is variable, so we'll need a way to store the energy in a more useful form. For this, we need

The storage battery. We'd really like to just hook the solar cells to the cell phone and call it a day, but this doesn't work because much of the time they can't put out enough instantaneous power. So we'll store the solar energy in an intermediate battery which can provide high power.

The solar controller. This circuit detects when the batteries have stored up a useful amount of energy, and turns on the cell phone charging regulator. Then it monitors the amount of energy in the storage battery, and turns off when the storage batteries are depleted.

The DC-DC converter. The solar system runs at a variable voltage between 2v and 3v. To charge the battery we need a regulated 5V DC. This is accomplished by using a DC-DC converter, in this case a lightweight universal converter called an AnyVolt Micro.

When I initially envisioned the charger section, I had planned to do it the easy way: a solar panel feeding a regulator which would trickle charge the battery. Unfortunately, Motorola decided that this wouldn't be possible - my Motorola V325 phone requires at least 100 milliamps of charge current, or it will continually cycle the charging circuitry on and off. This actually depletes the battery rather quickly instead of charging it, which is the opposite of what we want. In full sun, the solar cells I'm using provide 3V and 150 milliamps when wired in parallel. This isn't enough power to sustain a 100 milliamp charge current under real-world conditions, where we have cloud cover, tree cover and time of day to contend with.

This is the exact same problem that full-scale solar power systems have to contend with: peak power demands often exceed the abilities of the solar cells. We're going to solve the problem in the same way: by using a storage battery. In this case, I've used two AAA NiMH batteries in series. These are charged at all times by the solar cells through a diode. The power output of the solar cells is moderate enough that we don't need to add any other circuitry to prevent overcharging the batteries.

However, we will need a circuit to start charging the cell phone when the storage batteries are charged, and stop charging the cell phone when the storage batteries are depleted. This will prevent the cycling problem - the batteries will charge up for, say, 20 minutes at 100 milliamps, until they've reached a high enough state of charge to deliver an appreciable amount of energy to the cell phone. Then the cell phone will charge from the storage battery at, say, 250 milliamps (the actual charge current depends on how dead the cell phone's battery is) for ten minutes, and then with the storage battery depleted, shut charging off for the next twenty minutes until the storage batteries are charged again. In this way, we charge the cell phone at an acceptable charge current even though the solar cells can't put out enough power continuously.

An advantage of having a storage battery in the system is that you can leave the solar charger outside or in a window to collect energy during the day, and then charge your phone using that stored energy in the evening after the sun has set. This makes it possible to use the solar charger as your everyday cell phone charger, instead of being for emergency or off-the-grid use only.

Another advantage of having a storage battery is that it means the solar charger can function as a solar battery charger for AAA cells. This is useful if you're carrying, say, a digital camera that runs from AAA cells. If you needed AA batteries instead, use an AA battery holder in place of the AAA holder in the instructions.

The last nice thing about this system is that in an absolute emergency, say, after an alien invasion has blotted out the sun, you could replace the rechargeable NiMH batteries with disposable alkalines and charge your cell phone from them. In most circumstances you'd never need this capability, but if you have to contact the president regarding the aliens' one weakness, it might just save the planet.

This is ideal for campers or travelers. I bought a high capacity <a href="http://www.freenergyshop.com/category/sun/solar-chargers" rel="nofollow">solar charger</a> and I was surprised how long it stores energy. The concept of a waterproof solar charger is new to me, do they convert less solar energy due to the protection from water? Thx. for this thoughtful post!

VBT (TM) LOL!!! Made me chuckle, thank you Anti-Martha

how can i buy it the link doesnt work and also how much is it?

<a href="http://www.espow.com/wholesale-batteries-chargers-solar-chargers.html" rel="nofollow">http://www.espow.com/wholesale-batteries-chargers-solar-chargers.html</a> There are many similar devices maybe you can find one useful for you,or you can go to <a href="http://www.amazon.com/" rel="nofollow">amazon</a> to have a look.

hello?! anyone there? 1. how much were the solar panels? 2. what company did you buy them from? 3. how much did this project cost?

I'm going on a 7 day backpacking trip later this summer and needed a portable solar charger to charge my camera. Thanks you so much, this will help out a LOT!

how much did this cost? I'm on a budget. and would you have to actually make the bag? I have mad sewing skills, but I would think that a seal line brand map bag would work well and maybe be more waterproof. how waterproof is it really? I'm a sea kayaking guide and this would be pretty awesome to charge on the deck of my boat. I run out of battery on my phone by the end of a 5 day trek.

This may be a stupid question, but how can I figure out how much current my phone needs to charge? I have a motorola with a 3.7v battery.<br />

Usb is always 5v. Or if not, check your wall wart to charge your phone. It will tell you the required output.<br />

&nbsp;Was just wondering if the waterproof pouch affects the efficiency of the solar panel all that much is or is the loss negligible?

can it be done for acid lead battery it will help me thaks

Can i use single op-amp because i don't have one of this?

love the ible you did great i have a question would it not just be easier to use the solar pack to charge some AA batteries and buy a little cell phone charger from walmart they run on 2 AA bateries and then just charge 2 AA batteries and put them in the them in the portable cell phone charger and bing bang boom done then just put it all away in your pouch when your done and yay your not lost in the wilderness

how mich is one and where can i get one!! :D:D great idea

Any chance on getting a premade circuit?

I'm building one of these as a project for school, and one thing I noticed is that the AnyVolt Micro output cuts out when the input is dropped below 2.2V, which, if I'm not mistaken, means that you could work the whole setup down to a Schottky diode and the AnyVolt Micro as it would automatically cut off the batteries from outputting if they drop below 2.2V. I ahven't finished testing the AVM yet so I'm not 100% sure of this yet though.

One issue is the quiescent draw of the AnyVolt Micro - it'll run the battery down in dim light just staying on. The other thing the comparator circuit does for you is provides for burst charging - my cell phone turns the backlight on when it starts charging, which draws significant power. By storing up the energy then charging at a higher rate, then turning off to recharge the batteries, the amount of total power consumed by the phone's backlight is reduced. The comparator adds hysteresis, which is important with those limitations.
Incedentally, since I wrote this Dimension came out with another regulator called LVBoost which is smaller, higher current, and can run down to .7v input, plus has significantly lower quiescent draw. You can get more juice out of the battery that way - like you say, the AVM cuts out at 2.2v, which works, but it only uses about half the capacity of the battery before cutting out to recharge. If I was doing this application today that's what I'd use.

Gotcha, I'd neglected the quiescent draw. I don't have the issue of the backlight coming on with my phone, but I added a USB port for my MP3 player, which does turn the screen on when it charges (I'm tempted to reconnect it's hold switch to the backlight to save power).
Oh wow that would be sweet, although I think my instructor might get mad as we just spent a bunch of money ordering AVM's haha. I'll keep that in mind if I do a v2.0 though.

Oh dear, just wanted to inform that if you don't clean pcb's copper surface with steelwool(?(sorry I'm Finnish and don't know what's the word for that)) it may block the electricity.

where did u buy your solar panels?

Thought another stickler would have mentioned it:<br/>Step 18 <strong>Testint</strong> Testing 1 2 3<br/><br/>Still a great ible<br/>

lol
step one; Software

Wow. Cool project indeed. Is there any logic to adding a diode to protect from the battery draining through the solar cells in low light situations (ie battery voltage higher than panel voltage)?

yes yes yes. without the diode the current will run back into the panels. You are correct.

Leaving the diode out could also damage the solar cells, not just drain the batteries.

I have a newbie question. Most of the diy solar projects I've seen you have to make sure that regulators diodes etc. are used and that you cut off one end of a phone charger and solder everything together. Instead of cutting off the wires from a charger, couldn't you just hook the solar cells directly to the charger? I figured that the charger has all of the necessary electronics already.
I'm new to soldering and electronics so forgive my lack of knowledge.

You can snip the wires off a car cigarette adapter and then connect the cigarette adapter to your solar panels, and then use that to charge your phone. You are right. The adapter that normally connects to the car battery has regulators and diodes and all of that stuff included. It takes a 12 volt charge from the battery and converts it to 5volts so you can safely charge your phone. <br/><br/>Although, its probably a good idea to install a diode between the solar panels and the cigarette adapter because I have seen instances where cheap cell phone battery chargers still drain the battery of the cell phone even when they are plugged into a cigarette adapter when the car isn't running. Diodes are super cheap and easy to install, so it shouldn't be too difficult. <strong>The regulator IS the cigarette adapter</strong> that changes the voltage to a normal level for your cell phone.<br/><br/>The black box or end of the connector contains the transformer, diodes, and anything else that would prevent you from overcharging or damaging your battery pack. <br/>

nice but id use D cells and you can also use solar panels from old calculators

<em>&quot;The last nice thing about this system is that in an absolute emergency, say, after an alien invasion has blotted out the sun, you could replace the rechargeable NiMH batteries with disposable alkalines and charge your cell phone from them. In most circumstances you'd never need this capability, but if you have to contact the president regarding the aliens' one weakness, it might just save the planet.</em><br/><br/>lol<br/>Thanx for the instruction i had in plans to make a exactly similar last summer but now it will be finished'<strong></strong><br/>

has anyone got a plan/design to use the small panels from solar garden lights to make a larger panel that could charge a 12V battery? I got about 30 lights that I want to caniblaize the panels out of and make something larger. thanks.

It is doable, you could probably follow the instructions from makezine, their actual magazine had an article in it on using salvaged solar panels. If you want to get any significant source of charge though, you will need a large and efficient panel. I went to Canadian Tire and bought an 80 dollar solar panel which outputs 12 volts @ 400ma in direct sunlight. It works great, but to charge a 7ah battery, it takes like 2 days. Those little panels *might* squeak 400ma. <br/><br/>What I did was plug the panel into one of those car booster packs, let it charge all day, go to work... whatever. Then when I got home, I would run my laptop off of the charged booster pack through an inverter. (stay off my case about the waste of time going from DC-AC-DC, i know, this was an experiment) It worked great! The only thing i didn't do was charge the laptop off of the booster pack or run it at full CPU. These two things would kill the booster pack very fast! The booster pack usually died almost fully, I could have extended this charge by bypassing the extra step (DC-AC-DC). I was up north working and this was a little experiment I did to stop myself from going nuts with boredom... (no soldering for 2 MONTHS!)<br/>

i guess it is possible, but there might be a problem with all the wiring

Nice idea. I wonder how to mod this project so that i can supply energy to a microcontroller. Then, I would be able to make some nice projects to put at sunlight with no need for external power. (but, then... I guess the batteries and the electronic components should not be exposed to direct sunlight)

Sorry... didn't finish reading some of the posts already made! But I see where someone asked the same question, so I got my answer there! Thanks!

This is definitely a cool project! I'm not very tech savvy so this question might seem a little weird... but I'm assuming I can use the same concept to make one to plug in small kitchen appliances, etc.? Would I need to change anything about the design other than the bag? (don't think it is neccesary to put a toaster in a bag LOL). And a kinda off the subject question... does anyone know where I can find an instructable on making my own enclosed compost tumbler out of recyclables?

Hi,
Sounds great! How would you go about making one large enough (powerful enough) to run the new portable Microwaves to take aboard a boat? Or to run a small fan or toaster for fancier camping? Or to run a computer and a lamp during a power outage? Seriously, you have a great idea.

<a rel="nofollow" href="http://www.otherpower.com/popup.html">http://www.otherpower.com/popup.html</a> <br/>-describes considerations in designing a system of similar size/use.<br/>

Hi,
Well, the basic idea would be the same, but a microwave is serious power, at least a couple hundred watts. This project is running at most a solar charge power of .45W and 1.5W to the cell phone. That would be a scale-up of 100 times or more in terms of power. This would necessitate different components and construction techniques. Also, this project only turns the charger on when the battery is ready, which might be any time between "now" and "an hour from now" depending on the sun and the state of charge of the battery. That's necessary for cell phone charging, but a bit annoying when you want breakfast.
With a larger system you'd need a much larger solar panel, of course. Something rated to charge a 12V battery would be ideal. You'd need bigger batteries, a 12v lead acid deep cycle battery would be the usual route.
Really the application is different enough that it'd be a new instructable.

What a great idea to use an intermediate storage battery so you can save the solar energy! How much does this device weigh? Have you hiked with it? If so, how well does it work? Approximately how much did it cost for parts?
I love your idea too. It looks like a great project for anybody who has a cell phone and likes to get off the beaten path for a few hours or a few days. Plus, it could be an essential piece of backpacking equipment for a life-saving emergency. Thank you for taking time to make such an awesome instructable!

One of the best instructables ever. Not easy to do for common people but extremely well explained.

Thanks!

My only comments are that you could build your own dc/dc converter and charge controller, another comment is that your current scheme that when sunlight is low, the charge controller is going to suck energy from the batteries. if I could make a small suggestion, there is a Micro M+ charger which is commercial available, but if you can get you hands on an October 1999 QST its fully detailed how its implemented, that is a good starting point, while i don't think you care about low RF noise and the p-channel mosfet is not needed, some of the other schemes there can be used here. Also I would recommend being a little more careful with your solder job, use a little less or get a finer multicore solder, any solder touching a component case is considered a defect in the manufacturing world under any class of JSTD001D
also surface mount diodes typically cost 2-3x what their leaded companions cost, you can take a leaded diode and surface mount it quite easily and save yourself a few bucks.

Hey, thanks for the feedback. It'd be a simple enough matter for someone following this instructable to build a different DC-DC converter if they were feeling ambitious. I had the AnyVolt Micro in my parts box. The diodes were also parts box, reasonable substitutions would work.
The charge controller does draw energy from the battery in the "off" state, but the quiescent draw of the op amp is .17mA and the typical loss from the resistor network is about .12 milliamps. That's well below the self-discharge rate of even an excellent NiMH battery, so I wasn't too worried about it.

When backpacking and such, you should always always turn off your cellphone when not in use. Especially if you are out of range, (which you probably were 27 miles is far ) When you are out of range the cellphone is not on standby but it is constantly trying to connect to a system that does not exist, which kills the battery real quick. . There is no use in a cellphone that can't connect anyway. Penguins know this.<br/><br/>Peace,<br/>PengiuinMan<br/><a rel="nofollow" href="http://www.PenguinSongs.com">www.PenguinSongs.com</a><br/>

I love this project. I have too much going on right now, but will probably do this someday.
Thanks.